syntheses and spectroscopic studies on macrocyclic complexes of

ISSN: 0973-4945; CODEN ECJHAO

E-Journal of Chemistry

http://www.ejchem.net 2012, 9(2), 497-503

Syntheses and Spectroscopic

Studies on Macrocyclic Complexes of

Dioxomolybdenum(VI) with Furil as Precursor

D. P. RAOa*

H.S.YADAVb, A. K. YADAVA

b, SANJAY SINGH

c, and U. S. YADAV

d

aDepartment of Chemistry

D.A-V College, Kanpur 208001, Uttar Pradesh, India bDepartment of Chemistry, North Eastern Regional

Institute of Science and Technology

(NERIST), Nirjuli, Arunachal Pradesh, India cDepartment of Chemistry

M.G.P.G. College, Gorakhpur, Uttar Pradesh, India dDepartment of Chemistry

Rajendra College, Chapra, Bihar, India

[email protected]

Received 20 July 2011; Accepted 21 September 2011

Abstract: Due to their biological relevance, molybdenum catalyzed oxygen

transfer reactions have great interest. With this view, some

dioxomolybdenum(VI) complexes with general formula [MoO2(mac)](acac)2,

(where mac = tetraazamacrocyclic ligands derived from condensation of furil

with 1,2-ethanediamine or 1,3-propanediamine and their reaction with β-

diketones) have been synthesized using dioxometal ion as kinetic template.

The prepared complexes have been characterized by electrical conductance,

elemental analyses, infrared and electronic data. All the dioxomoly-

bdenum(VI) complexes have octahedral geometry with six coordination.

Keywords: Dioxomolybdenum(VI), Condensation, Macrocyclic complexes.

Introduction

During the past few years, a great deal of research efforts have been directed to study the

transition metal complexes of high denticity ligands with a view to obtain the metal

complexes of unusual configuration and coordination number1. A number of planner

tetradentate ligands capable of giving six coordinate complexes are reported2. Structures of

some of these complexes have been established by X-ray studies. However, the chemistry of

transition metal complexes with macrocyclic ligands incorporating four nitrogen donor

atoms have received less attention particularly in case of dioxomolybdenum(VI) cation3-6

.

D. P. RAO et al. 498

Molybdenum is one of the biologically active elements and its oxocations have been found

to be wider in redox active sites in many molybdoenzymes7 involved in nitrogen, sulfur and

carbon metabolism. The biochemical importance of molybdenum is due to its ability to

provide facile electron transfer pathways, a consequence of easy interconvertibility of

different oxidation states and to form bonds with nitrogen which are sufficiently strong

permit the existence of stable complexes. The significance of molybdenum in physiological

functions of oxomolybdoenzymes has been established7-10

where molybdenum is found to

coordinate to one or more terminal oxo-groups in each enzymes. Keeping the importance of

dioxomolybdenum(VI) cations in oxygen transfer reactions, a new series of macrocyclic

complexes of MoO2(VI) with ligands derived by the condensation of furil with

1,2-diaminoethane and 1,3-diaminopropane have been synthesized using different

β-diketones viz. acetylacetone, benzoylacetone, thenoyltrifluroacetone, and dibezoylmethane

as cyclising agent under kinetic template effect of oxomolybdenum centre. The complexes

have been isolated in solid state and their tentative structures have been assigned on the

basis of their elemental analyses, electrical conductance and spectral data.

Experimental

All the chemicals and the solvents used were of the reagent grade. Furil used was Aldrich

product. Dioxomolybdenum(VI) acetylactonate was prepared by standard method using

sodium molybdate and actylactone. The β-diketones viz. acetylacetone, benzoylacetone,

thenoyltrifluoroacetone, and dibenzoylmethane were SRL products and the diamines used

were of reagent grade products.

Analytical methods and physical measurements

Microanalysis of carbon, hydrogen and nitrogen for the complexes were done at central

research facility, NERIST, nirjuli-791 109, Itanagar, Arunachal Pradesh, India. Kjeldahl’s

method was employed to estimate nitrogen for the complexes. Molybdenum was estimated

gravimetrically after decomposing the complex with concentrated nitric acid by standard

method11

. Sulfur was estimated as barium sulfate

12. The standard technique of melting point

(uncorrected) determination using sulfuric acid bath was employed. The electronic spectra of

the complexes were recorded with the help of Beckmann DU-2 spectrophotometer and

c Φ10 Russian spectrophotometer instrument in the ranges 2000-185 nm and 700-400 nm.

The infrared spectra of the complexes (4000-200 cm-1

) were recorded in KBr on Perkin-

Elmer 621 and Beckmann Acculab-9 spectrophotometers.

In-situ preparation of dioxomolybdenum(VI) complexes with ligands derived by

condensation of furil with 1,2-ethanediamine or 1,3-propanediamine

Molybdenyl acetylacetonate (2 mmol) dissolved in methanol (20 mL) was added to a

refluxing solution of furil (2 mmol) and 1,2-etahnediamine (4 mmol) or 1,3-propanediamine

(4 mmol) in ethanol (20 mL). The mixture was allowed to react under mild reflux for 5 h,

when the color of the solution turned yellow. The solvent was removed under vacuo at room

temperature and the dark yellow color product was isolated. The complex was thoroughly

washed with methanol/ethanol mixture. Yield 75%.

In-situ preparation of macrocyclic complexes of dioxomolybdenum(VI)

Molybdenyl acetylacetonate (2 mmol) dissolved in methanol (20 mL) was added to a

refluxing solution of furil (2 mmol) and 1,2-ethanediamine or 1,3-propanediamine (4 mmol)

in ethanol (20 mL). The mixture was subjected to mild reflux for 5 hours, when the color of

the solution intensified and turned yellow. To this reaction mixture, an ethanolic solution

(10 mL) of acetylacetone (2 mmol) and glacial acetic acid (5 mL) were added. The reaction

mixture was refluxed for about 5 h then yellow precipitate was obtained. The complex was

Syntheses and Spectroscopic Studies on Macrocyclic Complexes 499

purified by washing with the mixture (10 mL) of methanol/ethanol (1:1). Yield 65%. The

same procedure was adopted for the synthesis of other dioxomolybdenum(VI) macrocyclic

complexes using benzoylacetone, thenoyltrifluroacetone, and dibenzolylmethane. The

physical and analytical data of the complexes are presented in Table 1.

Table 1. Physical and Analytical data of the complexes.

Complex

Em

pir

ical

Fo

rmu

la

Dec

om

po

sin

g

Tem

per

atu

re,

OC

C%

Calc

d.

(fo

und

)

H%

Cal

cd.

(fo

und

)

N%

Cal

cd.

(fo

und

)

Mo

% C

alcd

.

(fo

und

)

S%

Cal

cd.

(fo

und

)

[MoO2(L1)]

(acac)2 C24H32N4MoO8 305

48.0

(47.8)

5.4

(5.3)

9.3

(9.3)

16.0

(15.9)

[MoO2(L2)]


49.7

(49.6)

5.8

(5.3)

8.9

(8.8)

15.3

(15.3)

[MoO2(mac1)]


52.4

(52.3)

5.5

(5.3)

8.4

(8.3)

14.4

(14.3)

[MoO2(mac2)]


56.2

(56.1)

5.3

(5.2)

7.7

(7.7)

13.2

(13.1)

[MoO2(mac3)]

(acac)2 C32H33N4MoO8SF3 306

48.9

(48.8)

4.2

(4.1)

7.1

(7.0)

12.2

(12.1)

4.1

(4.0)

[MoO2(mac4)]


59.4

(59.3)

5.1

(5.0)

7.1

(7.0)

12.2

(12.1)

[MoO2(mac5)]


53.8

(53.7)

5.8

(5.7)

8.1

(8.1)

13.9

(13.8)

[MoO2(mac6)]


57.3

(57.2)

5.6

(5.5)

7.4

(7.3)

12.7

(12.6)

[MoO2(mac7)]

(acac)2 C34H37N4MoO8SF3 306

50.1

(50.0)

4.6

(4.5)

6.9

(6.8)

11.8

(11.7)

3.9

(3.8)

[MoO2(mac8)]


60.0

(59.9)

5.4

(5.3)

6.9

(6.8)

11.8

(11.7)

Where, L1 = Ligand derived by condensation of furil with 1,2-ethanediamine (1:2); L2 = Ligand

derived by condensation of furil with 2,3-propanediamine (1:2); Mac1 = macrocyclic ligand derived

by condensation of L1 with acetylacetone; Mac2 = macrocyclic ligand derived by condensation of L1

with benzoylacetone; Mac3 = macrocyclic ligand derived by condensation of L1 with

thenoyltrifluoroacetone; Mac4 = macrocyclic ligand derived by condensation of L1 with

dibenzoylmethane; Mac5 = macrocyclic ligand derived by condensation of L2 with acetylacetone; Mac6

= macrocyclic ligand derived by condensation of L2 with benzoylacetone; Mac7 = macrocyclic ligand

derived by condensation of L2 with thenoyltrifluoroacetone; Mac8 = macrocyclic ligand derived by

condensation of L2 with dibenzoylmethane.


Results and Discussion

The dioxomolybdeum(VI) complexes were synthesized using in-situ method by refluxing

the reaction mixture of furil, diamines and molybdenyl acetylacetonate in 1:2:1 molar ratio

in aqueous ethanol. The reaction appears to proceed according to the following reaction

scheme:

O

O

O

O

+

NH2

NH2

NH2

NH2

1,2-ethanediamine

+

1,3-propanediamine

or

Furil

-

-

Molybdenyl acetylacetonate (acac)2O

(H2C)n (CH2)n

H2N

O O

N N

NH2

Mo

O

Ethanol

+ 2H2O

[MoO2(L)](acac) 2

Type I

Where, Furil + 1,2-ethanediamine = L1, Furil + 1,3-propanediamine = L

2. The parent

complexes [MoO2(L)](acac)2 react with β-diketones to yield [MoO2(mac)](acac)2 as given

below:

-

-

(acac)2O

(H2C)n (CH2)n

H2N

O O

N N

NH2

Mo

O +

R R

O O

[MoO2(L)](acac)2

O O

N N

NN

Mo

O

RR

(acac)2O(H2C)n (CH2)n

[MoO2(mac)](acac)2

2H2O+-Diketones

Type II

Where mac = tetraazamacrocyclic ligands derived from condensation of L1 or L

2 with β-

diketones in presence of dioxmolybdenum(VI) cation.

The elemental analysis (Table 1) of complexes show 1:1 metal to ligand stoichiometry.

The molar conductivity of dioxmolybdenum(VI) complexes in dimethylformamide showed

values of ΛM between 125-140 ohm-1

cm2 mol

-1 which indicate their electrolytic nature.

R R’ β-Diketone

CH3 CH3 Acetylacetone

C6H5 CH3 Benzoylacetone

C4H3S CF3 Thenoyltrifluroacetone

C6H5 C6H5 Dibenzoylmethane


Infrared spectra

The characteristic infrared spectral bands for the complexes are listed in Table 2. The

macrocyclic complexes of dioxomolybdenum(VI) exhibit >C=N absorption around 1620 -

1614 cm-1

, which normally appears at 1660 cm-1

in free ligands.13-15

The lowering of this

band in the complexes (Type - I) indicates the coordination of nitrogen atoms of the

azomethine groups to the molybdenum13-16

. The presence of a band at around 300 cm-1

may

be assigned to ν(Mo-N) vibration17

. The appearance of >C=N band and the absence of the

>C=O band around 1700 cm-1

is a conclusive evidence for condensation of the diamines

with the two keto group of furil.18

The bands appearing at 3340 and 3175 cm-1

may be

assigned to asymmetrical and symmetrical N-H stretching modes of the coordinated terminal

amino group.18

The dioxomolybdenum(VI) complexes prefer to form a cis-dioxo group due

to the maximum utilization of the d-orbital for bonding.. The cis-dioxo configuration in

MoO2(VI) moiety19-21

is characterized by two infra-red bands of νasym(O=Mo=O) and

νsym(O=Mo=O) in C2V symmetry. The presence of two infra-red bands in the 900-912 cm-1

and 930-940 cm-1

regions are assigned to νasym(O=Mo=O) and νsym(O=Mo=O) vibrations

respectively. The bands appearing at 1555 cm-1

and 1510 cm-1

are assigned to ν(C=O) and

ν(C=C) vibrations of acetylacetonate group present in outer coordination sphere22

. The

infrared spectra of macrocyclic complexes of type-II show the same pattern of bands but the

asymmetrical and symmetrical N-H stretching modes of terminal amino groups disappear

due to condensation of these amino groups with carbonyl group of β-diketones in cyclization

reactions23-24

.

Table 2. Infrared spectral bands of complexes.

Complex

Bands (cm-1

)

(1) (2) (3) (4) (5) (6) (7) (8)

[MoO2(L1)](acac)2 1620 300 1555 1510 900 930 3340 3175

[MoO2(L2)](acac)2 1620 302 1554 1510 902 932 3342 3176

[MoO2(mac1)](acac)2 1620 300 1555 1512 904 940

[MoO2(mac2)](acac)2 1618 301 1556 1510 903 940

[MoO2(mac3)](acac)2 1614 300 1560 1510 906 938

[MoO2(mac4)](acac)2 1620 302 1554 1513 908 940

[MoO2(mac5)](acac)2 1620 302 1552 1512 910 940

[MoO2(mac6)](acac)2 1618 301 1555 1512 903 940

[MoO2(mac7)](acac)2 1614 302 1554 1513 903 938

[MoO2(mac8)](acac)2 1620 300 1552 1510 905 940

Where, (1) ν (>C=N); (2) ν (Mo-N); (3) ν (C=0) of acetylacetonate group; (4) ν ( >C=C<) of

acetylacetonate group; (5) νasym (O=Mo=O); (6) νsym (O=Mo=O); (7) νasym (N-H); (8) νsym (N-H).

Electronic spectra

These spectra are similar to other dioxomolybdenum(VI) complexes involving

nitrogen donor atoms. The electronic spectra of the complexes were recorded in

10-3

mol L-1

solution in DMF and these spectral bands are interpreted according to

earlier reported energy levelscheme25–26

. The high intensity peaks observed in the

region 290-355 nm of the dioxomolybdenum(VI) complexes seem to be appeared due

to intra ligand n → π*/ π → π* transitions. A medium intensity peak appearing in the

region 342 nm and 390 nm may be assigned for ligand to metal charge – transfer

transition between the lowest empty molybdenum d-orbital and highest occupied

ligand molecular orbital20

.

The above details support the tentative structures of


dioxomolybdenum(VI) complexes of type(I) and macrocyclic complexes of the type (II) as

shown in the figures.

Conclusion

The spectral data show that the Schiff base condensation of furil, a versatile chelating agent,

with diamines and their cyclisation reaction with β-diketones are achieved by virtue of

kinetic template effect of dioxomolybdenum(VI) cation in aqueous ethanol medium. Schiff

bases behave as tetradentate ligands by bonding to the metal ion through the azomethine

nitrogen atoms. The analytical data show the presence of one metal ion per ligand molecule

and suggest a mononuclear structure for complexes. The analytical and electronic data

support the octahedral structure for MoO2(VI) complexes.

Acknowledgment

The authors are thankful to the Director, NERIST, Nirjuli, Itanagar, Arunachal Pradesh,

India for providing laboratory facilities for synthetic work and central research facility for

microanalysis of carbon, hydrogen and nitrogen.

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